(1) Field of the Invention
The present invention concerns a smoke detecting device and a method of detecting smoke. Embodiments of the invention may be applied to the detection of a fire by detection of fine particles and aerosols contained in smoke emitted by a fire.
(2) Description of Related Art
Smoke is one of the first indicators of the presence of a fire. Consequently, detection of smoke may be used to alert people of the presence of a fire in a building enabling them to escape the effects of a fire in time or to put out the fire. Moreover, in the case of nocturnal fires a smoke sensor fitted with an audible alarm may awaken and warn inhabitants of the presence of a fire.
In order to detect the presence of smoke two different types of physical phenomena are typically used. The first type of physical phenomenon involves the diffusion of light by the smoke, dust or associated aerosols. The second type of physical phenomenon exploited in the detection of smoke is the modification by the presence of smoke particles of the speed of movement of ions in the presence of an electric field.
In the case of optical smoke sensors based on the diffusion of light, two operating modes are generally implemented. In the first operating mode, smoke particles block the light between an emitter, for example a light emitting diode (LED) and a receptor, for example phototransistor. The subsequent reduction of light reaching the receptor triggers an alarm. Such sensors are often referred to as reduction optic sensors. In the second operating mode, which tends to be more commonly used, smoke particles diffuse light coming from the emitter and a part of the diffused light is received by a receptor. The signal received by the receptor is then small in absence of smoke but increases significantly in the presence of smoke. When the scattered light reaches the receptor, an alarm is triggered. Such smoke sensors are often referred to as diffusion optic sensors.
Although optical smoke sensors respond rapidly to smouldering fires, reduction optic sensors are efficient only when there is a large distance between the light emitter and the light receptor while diffusion optic sensors are not efficient for the detection of combustion gas or dark smoke which are the first signs of a fire. Moreover light emitters require a substantial electrical power supply, which can be a serious drawback when the sensor operates on batteries, leading to a reduced operating time.
Smoke detectors based on the movement of electrically charged particles, or ions, comprise a source of ionization for the generation of charged particles and a drift chamber in which the charged particles can slowly move under the influence of an electric field between electrodes polarized at low voltage. The movement of the ions in the electric field created between the electrodes generates a measurable continuous electric current. When smoke particles enter the drift chamber, they attach to the charged particles, neutralize them or reduce their speed of movement resulting in a significant drop in measured current between the electrodes. The drop in current triggers an alarm. This type of smoke detectors is generally referred to as ionization detectors.
Ionization detectors tend to respond more rapidly and with a reduced amount of smoke particles compared to optical sensors to flaming fires emitting smaller particles. In order to create charged particles, radioactive materials such as americium-241 were previously used. However, owing to new standards, commercialization of such smoke detectors is restricted because of the presence of the radioactive source in the smoke detector.
Another way of creating the charged particles is to use an electrical discharge such as those referred to as corona discharges. In such a way it is possible to generate ions without using a radioactive source. However such devices require a relatively high voltage to produce the ionization of the surrounding gas (air) which, even with a low ionic current (but a high voltage), requires a relatively high current at the power supply level since it is at a low voltage. This again can be a serious drawback when the sensor operates on batteries, leading to a reduced operating time. Moreover the current due to a corona discharge depends significantly on parameters such as air pressure, humidity, etc which can be another serious drawback. Indeed, in a traditional ionic smoke detector the current is measured and an alarm is triggered when the measured current changes by a given percentage. As a consequence it is difficult to detect the difference between the presence of smoke or a change of environmental conditions.